The present invention relates generally to the display of data by output devices, and more particularly to a method and apparatus for reducing memory storage requirements when displaying data on an output display device.
A computer system can output data to a wide variety of output display devices. Output display devices such as laser printers, plotters, and other printing devices produce an image or "visual representation" onto a sheet of paper or the like, while output display devices such as computer monitors develop visual representations on a computer screen.
Many output display devices receive display data in the form of a "bitmap" or "pixel map" and generate visual representations from the display data. A pixel is a fundamental picture element of a visual representation generated by a display device, and a bitmap is a data structure including information concerning a number of pixels of the representation. Bitmaps that contain more than on/off information are often referred to as "pixel maps." As used herein, both bitmaps and pixel maps are referred to as "bitmaps."
A printer can print dots on a piece of paper corresponding to the information of a bitmap. Alternatively, a computer monitor can illuminate pixels based upon the information of the bitmap. A "raster" output device creates a visual representation by displaying the array of pixels arranged in rows and columns from the bitmap. Most output devices, other than plotters, are raster output devices. Typically, a "page" of pixels corresponding to a printed or displayed page is received and stored in memory before the pixels are displayed by the output display device.
A visual representation can contain a number of image types, including text, graphics, photographic images, etc. Data of these types can be efficiently stored in files with other image information as high level "objects." An "object", as referred to herein, is the data and attributes defining a particular visual representation. The objects can be edited or otherwise manipulated using an application program ("software") running on a computer. When displaying the objects with an output display device such as a printer or display screen, the objects are typically first rasterized (or "rendered") into bitmaps. The output display device stores display bitmap data in memory before displaying the data.
A problem in the prior art methods of providing bitmaps to output display devices is that a large amount of storage space is required to store the bitmap before it is displayed. The requirements for storage space have become greater as the desire for high-resolution representations with more realistic attributes has become more prominent. For example, using a laser printer capable of printing black-and-white images at a resolution of 600 dots per inch (dpi), a typical displayed page requires about 3.8.times.10.sup.6 bytes of memory. When printing a page of color pixels, for example, having 8 bits per color per pixel, the memory requirement increases to about 121.times.10.sup.6 bytes of memory. With such memory requirements, a significant portion of the cost of manufacturing an output display device such as a laser printer is the cost of the required memory.
A method that has been used to reduce the memory requirements for displaying high-resolution images involves the compression of the bitmap data according to a compression method or algorithm. A compression algorithm can significantly reduce the space needed to store bitmaps by removing information from bitmaps or other objects. Some compression algorithms are "lossless", meaning that they compress data and reduce storage requirements with no loss of essential information. This type of compression is often used with text objects and the like, since text character codes often include extra information unrelated to the identity of the text characters. Other types of compression algorithms are "lossy", meaning that they compress data with some loss of information. These types of compression algorithms are typically used with image bitmap data, since the loss of information can often be noticeable in a high resolution image. When the compressed bitmap is to be displayed, it is decompressed using a corresponding decompression algorithm and sent to the print engine, monitor, or other output display device.
A problem with the compression method of the prior art occurs when different types of objects are to be displayed by an output display device. For example, a page of data can include text objects such as words or paragraphs, graphics objects such as bar charts or geometric shapes, and image objects such as a digitized photograph. A compression algorithm that is good for text objects may, for example, be less than adequate for image objects, and vice versa. For example, lossy compression techniques may be adequate for image objects in that they can highly compress the image object data, but may be less than adequate for text objects, where the lost data would be apparent. A lossless compression technique is good for text objects, but may not adequately compress image objects. Thus, the selection of a single compression algorithm will almost always result in a less-than-optimal compression of mixed object types.